In general, polyamide resins (polyamides) are readily oxidizable, thermally unstable polymers. Polyamides are discolored to yellow when being heated in the presence of oxygen, and further discolored to brown or black when undergoing a severer thermal history. Discolored polyamides do not exert their original functions, and thus have low commodity value, whereby these are merely used in black-pigmented molding materials. Brown or black discoloration of polyamides is called “scorch” and such polyamides are excluded from commercial products and should be discarded. In order to prevent yellow discoloration of polyamides during a preparation process (polymerization process), an antioxidant (thermal stabilizer) is generally added to a reaction system during polycondensation in a molten state or preparation of a material (a nylon salt aqueous solution). The most widely used antioxidants for polyamides are phosphorus acid compounds such as hypophosphite salts and phosphite salts. These phosphorus acid antioxidants are oxidized to phosphite salts or phosphate salts to remove oxygen from the polycondensation reaction system and thereby to prevent oxidative damage of polyamide molecules. Phosphite salts or phosphate salts, which are significantly effective for prevention of yellow discoloration and are inexpensive, are remarkably useful antioxidants for polyamides (see Patent Document 1).
Even when colorless polyamides that are not discolored to yellow are shaped into transparent films from a molten state, heterogeneous granules evidently differing from the surrounding polyamides can be visually observed in the prepared films in many cases. This suggests that polyamide molecules having melt viscosity (flowability) that is clearly different from that of normal polyamide molecules are generated in the films, although the molecular structure of the polyamides is not yet changed to an extent that any functional group absorbing visible light is generated. These heterogeneous granules that are clearly different from normal polyamide molecules are called gels or fish eyes. The primary cause of the occurrence of them is probably peculiar polymerization of polyamide molecules or non-linear molecular growth (formation of three-dimensional polymers).
When such polyamides containing a large number of gels or fish eyes are used in production of transparent or thin articles, such as bottles, sheets, films, and fibers, defective products occur with an extremely high frequency, thereby resulting in low productivity. Even if a filter that removes gels or fish eyes formed in molten polyamides is used before molding, microgels cannot be completely removed from polyamides. Furthermore, the filter must be replaced with new ones frequently due to an increase in pressure difference of the filter. This causes a reduction in continuous production time and thus productivity. Also, in the case that such polyamides are used as colored or crystallized molding materials with a relatively large thickness, the surface appearance thereof is likely to be damaged. Therefore, it is desirable that the number of gels and fish eyes in polyamides is as small as possible in any application.
It is known that when phosphorus acid antioxidants such as hypophosphite salts and phosphite salts absorb oxygen contained in reaction systems of polyamides and are oxidized into phosphite salts and phosphate salts, respectively, these acidified antioxidants act as catalysts for amidation reaction. This leads to not only difficulty in control of polycondensation reaction but also specific polymerization or non-linear molecular growth of the polyamide molecules, thereby forming gels or fish eyes. To prevent the formation of gels or fish eyes, alkaline components such as sodium hydroxide and sodium acetate are added simultaneously into the reaction system. Alkaline components are generally called antigelling agents. Gels or fish eyes are formed by not only the existence of the phosphorus acid antioxidants, but also by other factors, such as the existence of other acidic impurities, existence of foreign objects having a physical action, and generation of radicals caused by thermal history. However, for polyamides obtained from diamine components that are mainly composed of m-xylylenediamine and dicarboxylic acid components that are mainly composed of adipic acid, the presence of the phosphorus acid antioxidant significantly affects the formation of gels or fish eyes. As a result, even neutralized phosphite salts or phosphate salts cannot sufficiently reduce the formation of gels or fish eyes.
Patent Documents 2, 3, and 4 disclose solid-phase polymers of poly(m-xylylene adipamide) in which melt polymers of poly(m-xylylene adipamide) having a specific terminal-group balance and relative viscosity are solid-phase polymerized so as to have a specific range of relative viscosity, or a combination of a specific range of relative viscosity and a specific number average molecular weight, in order to reduce the number of gels or fish eyes. Also, Patent Document 5 discloses control of a mixture of two or more polyamides so as to have a specific terminal-group concentration difference. However, none of these documents describes the amino-group reaction rate.
Patent Document 6 discloses a method of solid-phase polymerization of polyamide 6 in a water vapor atmosphere for the purpose of inhibition of the formation of gelled substances, but this document does not describe essential properties of the melt polymer fed for the solid-phase polymerization or antioxidant.
Patent Document 7 discloses a method of solid-phase polymerization of polytetramethylene adipamide in a water vapor containing atmosphere for the purpose of prevention of yellow discoloration. However, a prepolymer containing an excess amount of 1,4-diaminobutane is used as a starting material in this method, and this prepolymer is quite different from the polyamide resin containing an excess amount of carboxyl groups, which is the essential component of the present invention.
Patent Document 8 discloses an engineering polyamide composition in which an aromatic monocarboxylic acid or an aromatic primary monoamine both having a specific concentration and a hypophosphite salt having a specific concentration are incorporated into a polyamide composed of an aromatic dicarboxylic acid and a C6-C18 aliphatic alkylenediamine, and a method of production of the engineering polyamide composition. The monocarboxylic acid or monoamine and the hypophosphite salt are added for the purpose of an improvement in thermal aging resistance. However, this document does not describe the amino-group reaction rate.
Patent Document 9 discloses a polyamide composition comprising two or more polyamides so as to have a specific relationship between the terminal-group concentration difference of polyamides and the concentration of phosphorus atoms. This document, however, does not describe the amino-group reaction rate but teaches that the reduction of the concentration of phosphorus atoms is preferred.
As described above, with respect to polyamides obtained from diamine components mainly composed of m-xylylenediamine and dicarboxylic acid components mainly composed of adipic acid, resins satisfying both prevention of yellow discoloration and the reduction of gelatinization or fisheye formation and a method of preparing the polyamide are still unknown.
[Patent Document 1] Japanese Examined Patent Application Publication No. 48-23199
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2000-234021
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2000-234022
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2001-233958
[Patent Document 5] Japanese Patent No. 3458399
[Patent Document 6] Japanese Patent No. 3427446
[Patent Document 7] Japanese Unexamined Patent Application Publication No. 56-149431
[Patent Document 8] Japanese Examined Patent Application Publication No. 7-64978
[Patent Document 9] Japanese Patent No. 3419065